Rotary optical beam generator

ABSTRACT

An optical fiber device may include a unitary core including a primary section and a secondary section, wherein at least a portion of the secondary section is offset from a center of the unitary core, wherein the unitary core twists about an axis of the optical fiber device along a length of the optical fiber device, and wherein a refractive index of the primary section is greater than a refractive index of the secondary section; and a cladding surrounding the unitary core.

RELATED APPLICATION

This application is a continuation-in-part (CIP) of U.S. patentapplication Ser. No. 15/802,897, filed on Nov. 3, 2017, which claimspriority under 35 U.S.C. § 119 to U.S. Provisional Patent ApplicationNo. 62/425,431 filed on Nov. 22, 2016, the contents of which areincorporated by reference herein in their entirety.

This application claims priority under 35 U.S.C. § 119 to U.S.Provisional Patent Application No. 62/715,040, filed on Aug. 6, 2018,the content of which is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present disclosure relates to an optical fiber device for generatingan optical beam with an annular beam shape and, more particularly, to anoptical fiber device for generating a rotary optical beam with anannular beam shape directly in an optical fiber (i.e., without usingfree-space optics).

BACKGROUND

A beam profile of an optical beam has a significant impact on processingperformance associated with material processing performed using theoptical beam. For example, an optical beam with an annular beam profilecan enable superior metal cutting. However, beam profiles of mostfiber-delivered optical beams are relatively simplistic. For example,the beam profile may be a Gaussian or near-Gaussian profile for alow-beam-parameter-product (BPP) laser (e.g., a BPP of less than orequal to approximately 3 millimeters times milliradians (mm-mrad)) thatcan be used for processing thin sheet metal (e.g., sheet metal with athickness of less than or equal to approximately 3 mm) using a tightlyfocused optical beam. As another example, the beam profile may be atop-hat (sometimes referred to as a flattop) profile for a high BPPlaser (e.g., a BPP of greater than approximately 3 mm-mrad) that can beused for processing thick sheet metal (e.g., sheet metal with athickness greater than approximately 3 mm) using a larger beam.

SUMMARY

According to some possible implementations, an optical fiber device mayinclude a unitary core including a primary section and a secondarysection, wherein at least a portion of the secondary section is offsetfrom a center of the unitary core, wherein the unitary core twists aboutan optical axis of the optical fiber device along a length of theoptical fiber device, and wherein a refractive index of the primarysection is greater than a refractive index of the secondary section; anda cladding surrounding the unitary core.

According to some possible implementations, an optical fiber device,including a unitary core including a primary section, wherein theprimary section of the unitary core has a non-circular shape, whereinthe unitary core twists about an optical axis of the optical fiberdevice along a length of the optical fiber device; and a claddingsurrounding the unitary core.

According to some possible implementations, a method may include:receiving, by a rotator fiber, an optical beam at a first end of therotator fiber, wherein the rotator fiber includes a unitary core thattwists about an optical axis of the rotator fiber along a length of therotator fiber; at least partially converting, by the rotator fiber, theoptical beam to a rotary optical beam, wherein the optical beam is atleast partially converted to the rotary optical beam as a result of theunitary core being twisted about the optical axis; and outputting, bythe rotator fiber, the rotary optical beam.

According to some possible implementations, a method may include:fabricating a rotator fiber preform having a unitary core with arefractive index structure that angularly varies with respect to acenter of the rotator fiber preform; consolidating the rotator fiberpreform in order to create a consolidated rotator fiber preform;concurrently drawing and spinning the consolidated rotator fiber preformin order to create a spun rotator fiber; and tapering the spun rotatorfiber in order to create a tapered spun rotator fiber, wherein, withinthe tapered spun rotator fiber, the unitary core rotates about anoptical axis of the tapered spun rotator fiber along a length of thetapered spun rotator fiber.

According to some possible implementations, a method may include:fabricating a rotator fiber preform including a unitary core with arefractive index structure that angularly varies with respect to acenter of the rotator fiber preform; consolidating the rotator fiberpreform in order to create a consolidated rotator fiber preform; drawingthe consolidated rotator fiber preform in order to create a drawnrotator fiber; and twisting the drawn rotator fiber in order to create atwisted rotator fiber, wherein, within the twisted rotator fiber, theunitary core rotates about an optical axis of the twisted rotator fiberalong a length of the twisted rotator fiber.

According to some possible implementations, an optical fiber device mayinclude a core section that twists about an axis of the optical fiberdevice along a length of the optical fiber device, wherein a center ofthe core section is offset from the axis of the optical fiber devicealong the length of the optical fiber device, wherein a rate of twist atwhich the core section twists about the axis increases from a first rateof twist at a first end of the optical fiber device to a second rate oftwist at a second end of the optical fiber device, and wherein the coresection being twisted about the axis is to cause an optical beam,launched at the first end of the optical fiber device, to be at leastpartially converted to a rotary optical beam at a second end of theoptical fiber device; and a cladding surrounding the core section.

According to some possible implementations, a method may includereceiving, by a rotator fiber, an optical beam at a first end of therotator fiber, wherein the rotator fiber includes a core section thattwists about an axis of the rotator fiber along a length of the rotatorfiber such that a center of the core section is offset from the axis ofthe rotator fiber along the length of the rotator fiber, wherein a rateof twist at which the core section twists about the axis increases froma first rate of twist at a first end of the rotator fiber to a secondrate of twist at a second end of the rotator fiber; at least partiallyconverting, by the rotator fiber, the optical beam to a rotary opticalbeam, wherein the optical beam is at least partially converted to therotary optical beam as a result of the core section being twisted aboutthe axis; and outputting, by the rotator fiber, the rotary optical beam.

According to some possible implementations, an annular beam generatormay include an optical fiber device comprising a core section thattwists about an axis of the optical fiber device along a length of theoptical fiber device, the core section being offset from the axis of theoptical fiber device along the length of the optical fiber device,wherein a rate of twist at which the core section twists about the axisincreases along the length of the optical fiber device from a first endof the optical fiber device to a second end of the optical fiber device;and a cladding surrounding the core section.

According to some possible implementations, a method may includeobtaining a fiber preform including a core and a cladding surroundingthe core, the core being approximately centered on a central axis of thefiber preform; removing a portion of the cladding surrounding the corealong a length of the fiber preform; re-sleeving the fiber preform inorder to create a rotator fiber preform, wherein, within the rotatorfiber preform, the center of the core is offset from the central axis ofthe rotator fiber preform; and creating a rotator fiber using therotator fiber preform, wherein, within the rotator fiber, a center ofthe core is offset from the axis of the rotator fiber along a length ofthe rotator fiber, and wherein, within the rotator fiber, the coretwists about the axis of the rotator fiber along the length of therotator fiber.

According to some possible implementations, a method may includecreating an opening along a length of a cladding rod, the opening beingoffset from a central axis of the cladding rod; inserting a core rod inthe opening along the length of the cladding rod; consolidating the corerod and the cladding rod in order to create a consolidated rotator fiberpreform; and creating a rotator fiber using the rotator fiber preform,wherein, within the rotator fiber, a center of the core is offset froman axis of the rotator fiber along a length of the rotator fiber, andwherein, within the rotator fiber, the core twists about the axis of therotator fiber along the length of the rotator fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams of an overview associated with an examplerotator fiber for generating rotary optical beam described herein;

FIG. 2 is a diagram of an example environment in which a rotator fiberfor generating a rotary optical beam may be implemented;

FIG. 3 is a diagram illustrating example transverse near-field intensitypatterns of various low-order guided modes LP_(lm) of aparabolic-graded-index fiber below cutoff;

FIGS. 4A and 4B are diagrams of cross-sections of example rotator fibersfor generating rotary optical beams;

FIG. 5 is a diagram illustrating an example tapered rotator fiberdescribed herein;

FIG. 6 is a flow chart of an example process of a spun fiber techniquefor fabricating a rotator fiber described herein;

FIG. 7 is a flow chart of an example process of a twisted fibertechnique for fabricating a rotator fiber described herein;

FIGS. 8A-8C are diagrams associated with example simulations usingvarious taper lengths of the rotator fiber described herein;

FIGS. 9A and 9B are diagrams of additional example rotator fibers forgenerating rotary optical beams; and

FIGS. 10 and 11 are flow charts of example processes for fabricating arotator fiber with an offset unitary core, as described herein.

DETAILED DESCRIPTION

The following detailed description of example implementations refers tothe accompanying drawings. The same reference numbers in differentdrawings may identify the same or similar elements. The implementationsdescribed below are merely examples and are not intended to limit theimplementations to the precise forms disclosed. Instead, theimplementations were selected for description to enable one of ordinaryskill in the art to practice the implementations.

As described above, beam shapes of conventional fiber-delivered opticalbeams are relatively simplistic (e.g., having a Gaussian ornear-Gaussian profile, a top-hat profile, and/or the like). Generatingan optical beam with a comparatively more advanced beam shape, such asan annular beam shape (i.e., a ring-shaped beam) generally takesexpensive, specialized, alignment-sensitive free-space optics, such asaxicons, spiral phase plates, and/or the like. Moreover, such opticstypically need to be located in a processing head, distal to a fiberassociated with delivering the optical beam. The processing head is anopto-mechanical assembly that is subject to acceleration andcontamination (e.g., from smoke, metal debris, dust, and/or the like),and is therefore an undesirable location for expensive,alignment-sensitive, bulky, and/or heavy optical elements.

Further, conventional techniques for generating an optical beam with anannular beam shape typically provide an optical beam with poor beamquality. For example, the conventional techniques may generate anoptical beam with an excessively high BPP, an excessive amount of powerin a middle of the annulus, diffuse beam edges (e.g., with a relativelylong radial tail of power that causes poor processing quality), and/orthe like.

Some implementations described herein provide an optical fiber devicefor generating an optical beam with an annular beam shape directly in anoptical fiber (i.e., without any free-space optics). More specifically,the generated optical beam is a rotary optical beam (i.e., an opticalbeam that propagates in the optical fiber in a helical direction),thereby generating an optical beam with an annular beam shape. In someimplementations, the rotary character of the optical beam can bepreserved (e.g., when the optical beam exits the optical fiber) suchthat a laser spot projected from the optical fiber onto a workpiece, forexample, shows an annular beam profile with sharp edges and high beamquality. In this way, an optical beam with an annular beam shape may begenerated directly in the optical fiber, thereby facilitating improvedmaterial processing.

FIGS. 1A and 1B are diagrams of an overview associated with an example100 of a rotator fiber for generating a rotary optical beam, describedherein.

An optical fiber device for generating a rotary optical beam (referredto herein as a rotator fiber) may include a unitary core, which maycomprise a primary section and a secondary section, where at least aportion of the secondary section is offset from a center of the unitarycore. An example cross-section of such a rotator fiber is shown in FIG.1A. In the example shown in FIG. 1A, the secondary section is arrangedsuch that the secondary section (e.g., a section with a “+” shapedcross-section) separates the primary section into four sections. Asfurther shown, the rotator fiber may further include a cladding regionthat surrounds the unitary core.

As shown in FIG. 1B, the unitary core (i.e., the primary section and thesecondary section) may twist about an optical axis of the rotator fiber(e.g., a center of the unitary core) along a length of the rotatorfiber. In some implementations, the unitary core being twisted about theoptical axis causes an input optical beam (e.g., a non-rotary opticalbeam), launched at an input end of the rotator fiber, to be at leastpartially converted to a rotary optical beam at an output end of therotator fiber, as described in further detail below.

As further shown in FIG. 1B, in some implementations, the rotator fibermay be disposed between an input fiber and an output fiber. In someimplementations, the input fiber, the rotator fiber, and the outputfiber may be fusion-spliced together (e.g., using a conventional fiberfusion splicing technology).

In operation, the rotator fiber may receive an input optical beam fromthe input fiber. As shown, the input optical beam may include lightpropagating in one or more non-rotary guided modes. As the lightpropagates through the rotator fiber, and due to the twist of theunitary core along the length of the rotator fiber, the rotator fibergenerates a rotary optical beam from the input optical beam. In otherwords, the rotator fiber may at least partially convert the inputoptical beam to a rotary optical beam (e.g., by at least partiallyconverting the one or more non-rotary guided modes to at least onerotary guided mode and/or at least one rotary leaky wave). Thus, asindicated in FIG. 1B, the rotary optical beam may include lightpropagating in at least one rotary guided mode and/or at least onerotary leaky wave.

In some implementations, due to the light propagating in the at leastone rotary guided mode and/or the at least one rotary leaky wave, therotary optical beam has an annular beam shape. The rotary optical beamcan be launched via the output fiber (e.g., for use in materialprocessing, such as metal cutting). Here, the rotary character of theoptical beam can be preserved such that a laser spot projected from theoutput fiber shows an annular beam profile with sharp edges and highbeam quality. In this way, an optical fiber device may generate a rotaryoptical beam with an annular beam shape directly in an optical fiber(i.e., without any free-space optics), thereby facilitating improvedmaterial processing (e.g., as compared to the conventional techniquesdescribed above).

As indicated above, FIGS. 1A and 1B are provided merely as examples.Other examples are possible and may differ from what is described withregard to FIGS. 1A and 1B. For example, while the unitary core shown inFIGS. 1A and 1B comprises a primary section and a secondary section,other implementations are possible, such as a unitary core includingonly a primary section (e.g., a primary section with a non-circularshape, a primary section that is offset from the optical axis, and/orthe like). Additional details regarding example designs of the rotatorfiber are described below.

FIG. 2 is a diagram of an example environment 200 in which a rotatorfiber for generating a rotary optical beam may be implemented. As shownin FIG. 2, environment 200 may include an input fiber 210, a rotatorfiber 220, and an output fiber 230.

Input fiber 210 includes an optical fiber for launching an input opticalbeam (e.g., an input laser beam) to rotator fiber 220. In someimplementations, input fiber 210 may be a step-index optical fiber or agraded-index optical fiber, and may be designed to carry an optical beamnear an optical fiber axis of input fiber 210. In some implementations,input fiber 210 may be connected to an output fiber of a fiber laser, orinput fiber 210 can itself be an output fiber of a fiber laser.Alternatively, in some cases, the input optical beam may be launchedfrom free-space into input fiber 210. In such a case, input fiber 210may actually be omitted, and the input optical beam may be launchedstraight into rotator fiber 220 (e.g., rather than input fiber 210).

Depending on a system design and a design of input fiber 210, the inputoptical beam launched by input fiber 210 may be in the form of guidedmodes of a core of input fiber 210, in some implementations. In the caseof a step-index fiber, the guided modes may have a characteristichalf-divergence angle in air (θ) measured using the second-moment methodand satisfying the cutoff condition:sin(θ)<NA,where NA=√(n₁ ²−n₂ ²) is a numerical aperture and n₁ and n₂ arerefractive indices of the core of input fiber 210 and a cladding ofinput fiber 210, respectively. In the case of input fiber 210 being anon-step-index optical fiber, the guided modes may be definedanalogously using conventional solutions of a wave equation in fibers.

Whether input fiber 210 is a step-index optical fiber or anon-step-index optical fiber, the guided modes of weakly-guiding,circular-core fibers can be the so-called LP-modes, LP_(lm), where l,the rotational quantum number, is an integer that is greater than orequal to zero (l≥0), and m, the radial quantum number, is an integerthat is greater than or equal to one (m≥1). The upper limits of l and mmay be determined by the cutoff condition associated with therefractive-index profile of input fiber 210 described above.

In some implementations, the input optical beam launched by input fiber210 may be a single mode optical beam or a multi-mode optical beam, andmay be a polarized optical beam or an un-polarized optical beam. In acase where the input optical beam is polarized, the input optical beammay be circularly polarized since a circular polarization may be bettermaintained in rotator fiber 220 and/or output fiber 230 (e.g., ascompared to a linear polarization or an elliptical polarization). Insome implementations, if a linearly-polarized output optical beam isdesired, then the linear polarization may be generated from the circularpolarization after a termination of output fiber 230 using, for example,a quarter-wave plate.

Rotator fiber 220 includes an optical fiber device for at leastpartially converting an input optical beam, with a first rotationalstate, to an output optical beam with a second rotational state. Forexample, rotator fiber 220 may include an optical fiber device for atleast partially converting an optical beam (e.g., a non-rotary opticalbeam) to a rotary optical beam. In some implementations, rotator fiber220 may be relatively short in length (e.g., with a length of less than1m, but greater than 1 mm), whereas lengths of input fiber 210 andoutput fiber 230 may be dictated by the optical system in which rotatorfiber 220 is deployed (e.g., in a range from approximately 0.5 m toapproximately 100 m). Design aspects associated with the rotary opticalbeam generated by rotator fiber 220 are described in the followingparagraphs, while design aspects associated with rotator fiber 220 aredescribed below with regard to FIGS. 4A, 4B, 9A, and 9B.

In some implementations, the rotary optical beam may include lightpropagating in one or more rotary guided modes. Rotary guided modes aredefined as modes having l≥1 and one definite rotation direction. For amode to have one definite rotation direction is defined as follows. Formodes with l≥1, LP-modes can be expressed as modes with either sin(lϕ)and cos(lϕ) dependence, or e^(±(ilϕ))dependence, where ϕ is an angularcoordinate. Modes with l=0 have no angular dependence. The sine andcosine modes are standing waves in the angular direction, with angularnodes and with zero net rotation direction. The complex-exponentialmodes are angular traveling waves without angular nodes. These modeshave one definite rotation direction (e.g., clockwise orcounterclockwise), which is selected by the choice of (±) or (−) ine^(±(ilϕ)).

In some implementations, for the rotary guided modes in the rotaryoptical beams described herein, m may be equal to 1 (m=1) or may besignificantly less than l (e.g., less than approximately 50% of l, lessthan approximately 20% of l, and/or the like). In some implementations,using a comparatively low value of m (as compared to l) may ensure thatthe rotary guided mode will have a pronounced annular shape. Inparticular, a rotary guided mode with m=1 has no radial nodes other thanthe zero at the origin. In other words, the rotary guided mode with m=1is a single ring (whereas higher values of m correspond to rotary guidedmodes with m concentric rings). In some implementations, angulartraveling waves with one definite rotation direction, no angular nodes,and/or zero or few radial nodes may be generated by rotator fiber 220.

FIG. 3 is a diagram illustrating example transverse near-field intensitypatterns of various low-order guided modes LP_(lm) of aparabolic-graded-index fiber below cutoff. Modes of optical fibers withother rotationally-symmetric refractive-index profiles, such as astep-index optical fiber, may have similar intensity patterns as thoseshown in FIG. 3. In FIG. 3, both the angular-standing wave (cosine) andtraveling wave modes for each m are shown for l≥1, in the left and rightcolumns corresponding to each m, respectively.

In some implementations, rotary guided modes with m=1 (e.g., indicatedby the black box in FIG. 3) may be included in the rotary optical beamgenerated by rotator fiber 220. Notably, this set of rotary guided modesextends to higher values of l (e.g., to l=20 and higher). As shown, them=1 rotary guided modes have a pronounced annular shape with no nodes inany direction. In some implementations, rotary guided modes withslightly higher m (e.g., m=2, m=3, and/or the like) can also provide auseful annular beam, particularly for higher values of l. In someimplementations, one or more of the rotary guided modes, included in therotary optical beam, may have an l value that is greater than or equalto 10 (l≥10, such as l=15, l=18, l=20, and/or the like).

Additionally, or alternatively, the rotary optical beam may includelight propagating in one or more rotary leaky waves. Leaky waves are aclass of non-guided light in optical fibers (e.g., light that is notguided by the core of the optical fiber). Leaky wave light launched intoa core of an optical fiber may escape into cladding of the opticalfiber. However, in contrast to most non-guided light in fibers, theleaky wave light leaks relatively slowly from the core into thecladding.

Rotary leaky wave light, in particular, can have low loss over arelatively wide range of parameters. For example, in a step-index silicafiber with an NA of 0.10 and a core diameter of 50 micrometers (μm),rotary leaky wave light with wavelength of 1030 nanometers (λ=1030 nm)with no radial nodes and with a characteristic half-divergence angle θsuch that sin(θ)=0.11 has a calculated loss of only 0.14 decibels permeter (dB/m). Thus, while rotary leaky waves do not satisfy thecriterion for guided modes, rotary leak waves can be used inapplications with output fiber lengths on the order of tens of meters orless, such as passive optical power delivery fibers and active amplifierfibers, where losses of up to a few dB can be acceptable. Similar to thecase of rotary guided modes, rotary leaky waves have one definiterotation direction and no angular nodes, generally zero or few radialnodes, and may be included in the rotary optical beam generated byrotator fiber 220. In some implementations, one or more of the rotaryleaky waves, included in the rotary optical beam, may have an l that isgreater than or equal to 10 (l≥10, such as l=15, l=18, l=20, and/or thelike).

In some implementations, rotary optical beam may comprise a combinationof one or more rotary guided modes and/or one or more leaky waves. Insome implementations, in a case where the input optical beam is a singlemode optical beam, rotator fiber 220 can be designed such that therotary optical beam comprises a relatively pure (e.g., greater thanapproximately 50% purity, greater than approximately 80% purity, and/orthe like) single rotary guided mode or rotary leaky wave with aparticular value of l. In other words, in some implementations, rotatorfiber 220 can be designed such that at least 50% of input power,associated with the input optical beam, is converted to a single rotaryguided mode or a single rotary leaky wave in the output optical beam. Asdescribed above, the rotary optical beam (e.g., including one or morerotary guided modes and/or one or more rotary leaky waves) has anannular shape at an output end of rotator fiber 220.

Returning to FIG. 2, output fiber 230 includes an optical fiber forreceiving an output optical beam (e.g., a rotary optical beam) launchedby rotator fiber 220. In some implementations, output fiber 230 may be astep-index optical fiber, a graded-index optical fiber, or a fiber witha specialized index profile, such as an annular-core fiber that isdesigned to carry a rotary optical beam with minimal coupling into othermodes or leaky waves, and/or that is designed to provide a preferredradial intensity profile. In some implementations, output fiber 230 canbe omitted if, for example, an output of the system is to be coupleddirectly into free-space (e.g., rather than into fiber).

The number and arrangement of elements shown and described inassociation with FIG. 2 are provided as examples. In practice,environment 200 may include additional elements, fewer elements,different elements, differently arranged elements, and/or differentlysized elements than those shown in FIG. 2.

FIGS. 4A and 4B are diagrams of cross-sections 400 and 450,respectively, of example rotator fibers 220 for generating rotaryoptical beams.

As shown in FIG. 4A, in some implementations, rotator fiber 220 mayinclude a unitary core 405 that includes primary section 410 withrefractive index n₁ (e.g., sections 410-1, 410-2, 410-3, and 410-4 inthe example shown in FIG. 4A) and a secondary section 430 with arefractive index n₃. Unitary core 405 is described as unitary in thatthe sections of unitary core 405 (e.g., primary section 410 andsecondary section 430) touch one another such that the sections ofunitary core 405 form a single unit within rotator fiber 220. As furthershown, rotator fiber 220 may include a cladding 420, with a refractiveindex n₂, that surrounds unitary core 405. In some implementations, asillustrated in cross-section 400, secondary section 430 may be arrangedin unitary core 405 such that at least a portion of secondary section430 is offset from a center of unitary core 405.

In some implementations, unitary core 405 may twist around an opticalaxis of rotator fiber 220 (e.g., a center of rotator fiber 220) along alength of rotator fiber 220 (e.g., in the manner described above and asillustrated in FIG. 1B). In some implementations, a rate of twist aboutthe optical axis increases from a first rate of twist toward a first endof rotator fiber 220 (e.g., an end proximate to an input fiber 210) to asecond rate of twist toward a second end of rotator fiber 220 (e.g., anend proximate to an output fiber 230). For example, the rate of twisttoward an input end of rotator fiber may increase from zero or near zerotwists per mm (e.g., a rate of twist that is less than or equal toapproximately 0.02 twists per mm (approximately one twist per 50 mm)),to approximately 0.17 twists per mm (approximately one twist per 6 mm)or more toward an output end of rotator fiber 220.

In some implementations, rotator fiber 220 may be tapered such that asize (e.g., a diameter) of unitary core 405 substantially matches a sizeof a core of input fiber 210 and/or output fiber 230 at respective endsof rotator fiber 220.

FIG. 5 is a diagram illustrating an example tapered rotator fiber 220.As shown in FIG. 5, in some implementations, rotator fiber 220 may betapered such that a size of rotator fiber 220 at an input end of rotatorfiber 220 (e.g., an end spliced to input fiber 210, where the twist rateis at or near zero) is smaller than a size of rotator fiber 220 at anoutput end of rotator fiber 220 (e.g., an end spliced to output fiber230, where the twist rate is increased as compared to the input end).

As further shown in FIG. 5, a rate of twist at which unitary core 405twists about the optical axis may increase from a first rate of twist(e.g., a twist rate of zero or near zero) toward an input end of rotatorfiber 220 to a second rate of twist toward an output end of rotatorfiber 220. As indicated above, FIG. 5 is provided merely as an example.Other examples are possible and may differ from what is described withregard to FIG. 5. Although rotator fiber 220 is illustrated as beingstraight in FIG. 5, rotator fiber 220 may have any shape.

Returning to FIG. 4A, in some implementations, n₁ is greater than n₂ andn₃, and n₃ is greater than or equal to n₂ (n₂≤n₃<n₁). In other words, n₁is different from (e.g., greater than) n₃ and n₂, and n₃ may bedifferent from (e.g., greater than or equal to) n₂. This relationshipamong refractive indices of rotator fiber 220 facilitates generation ofthe rotary optical beam as light propagates through rotator fiber 220.For example, most of the input optical beam may be launched in primarysection 410 (with refractive index n₁), while a portion of the inputoptical beam may be launched in secondary section 430 (with refractiveindex n₃). Here, since both n₁ and n₃ are greater than n₂ (therefractive index of cladding 420), the light launched in unitary core405 (e.g., primary section 410 and secondary section 430) may be guidedby cladding 420. Further, since n₃ is less than n₁, secondary section430 somewhat guides the light in the separate sections of primarysection 410, and will twist the light about the optical axis of rotatorfiber 220 along the length of rotator fiber 220 as unitary core 405twists about the optical axis, thereby generating the rotary opticalbeam.

In some implementations, as illustrated in example cross-section 400,secondary section 430 may separate primary section 410 into at least twosections (e.g., such that secondary section 430 is between sections ofprimary section 410). In some implementations, unitary core 405 mayinclude a primary section 410 with at least two sections (e.g., twosections, three sections, four sections, six sections, and/or the like).In some implementations, at least two of the sections of primary section410 may have approximately equal cross-sectional areas. Additionally, oralternatively, at least two of the sections of primary section 410 mayhave different cross-sectional areas.

In some implementations, as illustrated in example cross-section 400, across-section of secondary section 430 may be symmetric with respect tothe optical axis of rotator fiber 220. Alternatively, a cross-section ofsecondary section 430 may be asymmetric with respect to the optical axisof rotator fiber 220, in some implementations.

In some implementations, secondary section 430 may comprise at leastthree portions, where the at least three portions extend in directionsthat are perpendicular to the optical axis of rotator fiber 220 in aplane of a cross-section of rotator fiber 220. In some implementations,a direction in which one of the at least three portions extends may beperpendicular to a direction in which another of the at least threeportions extends. For example, with reference to cross-section 400,secondary section 430 may include a horizontal portion, a first verticalportion (e.g., a vertical portion above the horizontal portion ofsecondary section 430 in FIG. 4A), and a second vertical portion (e.g.,a vertical portion below the horizontal portion of secondary section 430in FIG. 4A). Here, as shown, the horizontal portion, the first verticalportion, and the second vertical portion extend in directions that areperpendicular to the optical axis of rotator fiber 220 in a plane of across-section of rotator fiber 220. As further shown in FIG. 4A, thedirection in which the horizontal portion extends is perpendicular tothe direction in which the first vertical portion extends, and thedirection in which the horizontal portion extends is perpendicular tothe direction in which the second vertical portion extends.

Notably, example cross-section 400 is provided as merely as an example.Generally, unitary core 405 (e.g., comprising primary section 410 andsecondary section 430) may have a refractive index structure thatangularly varies with respect to the optical axis of rotator fiber 220,where unitary core 405 twists about the optical axis along the length ofrotator fiber 220. In example cross-section 400, the angularly varyingrefractive index structure is that of the “+” shaped secondary section430 in unitary core 405, surrounded by cladding 420. In this example,secondary section 430 forms complete dividers such that sections ofprimary section 410 are separated by secondary section 430.

Another example of the angularly varying refractive index structure mayinclude a rotator fiber 220 in which primary section 410 includes adifferent number of sections, separated by secondary section 430, thanshown in FIG. 4A. In some implementations, a symmetry of secondarysection 430, associated with the refractive index structure of unitarycore 405, may be selected based on a desired rotary guided mode to beincluded in the rotary output beam. For example, in case where therotary guided mode with l=8 is desired, then a symmetry of secondarysection 430 about the optical axis of rotator fiber 220 may be selectedsuch that the refractive index structure of unitary core 405 forms asymmetric eight-bladed divider (e.g., such that primary section 410comprises eight sections). Generally, the symmetry of secondary section430 may preferentially create modes with l that is equal the value of l,or a multiple thereof. For example, if rotator fiber 220 includes asecondary section 430 that forms a symmetric four-bladed divider (e.g.,such that primary section 410 comprises four sections, as shown incross-section 400), then modes with l=4 may be preferentially excited,as well as modes with l values that are multiples of four, such as l=0,l=8, l=12, l=16, and/or the like.

Yet another example of the angularly varying refractive index structuremay include a rotator fiber 220 in which secondary section 430 causesunitary core 405 to have an asymmetric cross-sectional shape withrespect to the optical axis of rotator fiber 220 (e.g., withoutseparating primary section 410 into multiple sections).

Still other examples of the angularly varying refractive index structuremay include a rotator fiber 220 in which primary section 410 and/orsecondary section 430 comprise a graded-index material, a rotator fiber220 in which secondary section 430 forms partial dividers (e.g., ascompared to complete dividers shown in example cross-section 400, suchas a secondary section 430 that spans approximately 85% of the innerdiameter of cladding 420, thereby forming unitary core 405 to include asingle, interconnected primary section 410), a rotator fiber 220including off-center round inclusions in unitary core 405, and/or thelike.

As another example, and as shown in example cross-section 450 of FIG.4B, rotator fiber 220 may not include secondary section 430, in someimplementations (e.g., rotator fiber 220 may not include any materialwith refractive index n₃). In other words, in some implementations,unitary core 405 may include only primary section 410. In such a case,the angular variation of the refractive index structure may be definedby a non-circular shape of primary section 410 within cladding 420(e.g., a five-pointed star shaped primary section 410 is shown inexample cross-section 450). Generally, a perimeter of the non-circularshape of primary section 410 may be at least partially concave (e.g.,the five-pointed star shaped primary section 410 includes five concaveportions). In such a case, the non-circular shape of unitary core 405may twist along the length of rotator fiber 220 (e.g., such that thepoints of the five-pointed star rotate about the optical axis of rotatorfiber 220 along the length of rotator fiber 220). Here, due to thenon-circular unitary core 405 twisting about the optical axis, lightpropagating in the non-circular unitary core 405 (e.g., lightpropagating in or near the points of the five-pointed star shown inexample cross-section 450) is twisted about the optical axis of rotatorfiber 220 along the length of rotator fiber 220, thereby generating therotary optical beam. In some implementations, rotator fiber 220including a non-circular unitary core 405 (i.e., a non-circular primarysection 410) may be tapered such that a size of unitary core 405substantially matches a size of a core region of input fiber 210 and/oroutput fiber 230 at respective ends of rotator fiber 220.

As indicated above, FIGS. 4A and 4B are provided merely as examples.Other examples are possible and may differ from what is described withregard to FIGS. 4A and 4B.

In some implementations, rotator fiber 220 with an angularly-varyingcross section can be fabricated using a rod-in-tube preform assemblymethod, whereby a rotator fiber 220 preform is fabricated (e.g., usingmultiple discrete pieces of glass, each with the appropriate refractiveindex). The rotator fiber 220 preform may then be fused together near amelting point of the glass. A twist can be implemented during a fiberdraw process using a preform spinning technique (e.g., similar to thatused in some polarization-maintaining, low-birefringence, orchirally-coupled-core fibers), or after the fiber draw process bytwisting a short length of rotator fiber 220 while heating rotator fiber220 (e.g., during fusion-tapering). Additional details regardingfabrication of rotator fiber 220 are described below with regard toFIGS. 6 and 7.

In operation, rotator fiber 220 may receive an optical beam at a firstend of rotator fiber 220. As the optical beam propagates through rotatorfiber 220, rotator fiber 220 may at least partially convert the opticalbeam to a rotary optical beam, and may output the rotary optical beam tooutput fiber 230.

In some implementations, modes of rotator fiber 220 follow the twistingpattern of the angularly varying refractive-index structure, meaningthat, as light propagates through rotator fiber 220, the modesinherently tend to have a rotary character. As a result, when rotatorfiber 220 is spliced into output fiber 230, light launched into outputfiber 230 may be in a rotary state comprising one or more rotary guidedmodes and/or one or more rotary leaky waves. The rate of twist (Φ, inunits of rotations per meter, for example) at an output end of rotatorfiber 220 determines an output divergence half-angle θ and anapproximate rotational state of the rotary optical beam according to thefollowing relations:sin(θ)˜2σn₁RΦl˜2σR sin(θ)/λwhere R is an effective radius of the rotary guided mode(s) and/orrotary leaky wave(s), typically being approximately 10% less than aradius of unitary core 405. Thus, for example, using a 100 μm corediameter rotator fiber 220 with a rotational pitch of 6 mm, a corerefractive index 1.450 (e.g., as is typical of fused silica glass), andan operating wavelength of λ=1080 nm, the effective radius isapproximately 45×10⁻⁶m (e.g., R˜90% ×(100/2)=45×10⁻⁶m). Here, the twistrate is 166.7 rotations per meter (e.g., 1/(6 mm)=166.7), and so itfollows that sin(θ)˜0.068 radians and l is approximately equal to 18(e.g., l˜18).

The rotational state of 18 describes a highly rotary beam, and theoutput divergence of ˜0.068 radians is typical of a fiber-deliveredlaser beam in industrial applications. The BPP is 3.1 mm-mrad (e.g.,45×0.068=3.1 mm-mrad), which is suitable for thin-metal processing,while with the annular beam shape is also suited for thick-metalprocessing.

As with any optical fiber, the light-guiding capability of rotator fiber220 is defined by the NA of rotator fiber 220, where NA=√(n₁ ²−n₂ ²).For the above example, in order to carry the rotary optical beam as arotary guided mode or as rotary guided modes, the NA of rotator fiber220 should be at least 0.068. Thus, the value of n₂ should be 1.4484 orless, as is achievable using, for example, doped fused silica.Alternatively, if it is desired to carry the rotary optical beam as arotary leaky wave, then a value of NA slightly smaller than 0.068 can beused (e.g., a value in a range from approximately 0.060 to approximately0.067). In some implementations, output fiber 230 should also have asuitable NA for conducting the rotary optical beam as rotary guidedmodes and/or rotary leaky waves.

In some implementations, the quality of coupling of input fiber 210 intorotator fiber 220 may determine how efficiently input power (e.g.,non-rotary) is converted into high-brightness rotary light power at anoutput of rotator fiber 220 (e.g., as opposed to being scattered out ofrotator fiber 220 or propagated as non-rotary light of degraded beamquality including, for example, many different modes). In order toensure such high-efficiency beam conversion, all transitions should besmooth and adiabatic, specifically in three aspects.

A first aspect associated with providing adiabatic transitions is thatcore sizes at transitions from input fiber 210 to rotator fiber 220 andfrom rotator fiber 220 to output fiber 230 should be substantiallymatching so that modes and/or leaky waves are transferred withoutsignificant mode scrambling. Thus, in a case where a core of input fiber210 and a core of output fiber 230 are different sizes, rotator fiber220 should be tapered such that a core size of rotator fiber 220 at aninput end and a core size of rotator fiber 220 at an output endsubstantially matches those of input fiber 210 and output fiber 230,respectively (e.g., as described above with regard to FIG. 5). In someimplementations, a rate of the taper may be gradual enough to enable anadiabatic transition. In some implementations, a square-root taperprofile may be used in order to enable a relatively short taper whilestill remaining adiabatic.

Another aspect associated with providing adiabatic transitions is thatthe rate of twist of rotator fiber 220 should be zero or near zero at aninput end of rotator fiber 220 (e.g., an end nearest to input fiber 210)and should gradually increase along the length of rotator fiber 220. Forexample, the rate of twist of rotator fiber 220 near input fiber 210 maycorrespond to a rotational state l of approximately 2 or less, 0.5 orless, and/or the like. In some implementations, the rate of twist mayincrease along the length of rotator fiber 220 to a maximum rate oftwist near an output end of rotator fiber 220 (e.g., an end nearest tooutput fiber 230). Here, the rate of change of the rate of twist shouldbe gradual enough to enable an adiabatic transition. Notably, the rateof twist does not go to zero or near zero near output fiber 230 (e.g.,since neither input fiber 210 nor output fiber 230 have anangular-varying refractive index structure there is no intrinsic amountof twist, and these fibers will transmit light with a given rotationalstate that is launched into these fibers (so long as the rotationalstate is below cutoff for that fiber)).

Still another aspect associated with providing adiabatic transitions isthat light that is directly launched into secondary section 430 (ifincluded in rotator fiber 220) from input fiber 210 should besubsequently captured by primary section(s) 410 so that this light alsoacquires rotary characteristics. In some implementations, this effectcan be achieved when a size of rotator fiber 220 is tapered upward frominput fiber 210 to output fiber 230. Thus, in order to satisfy the firstaspect described above, a core size of output fiber 230 should be largerthan a core size of input fiber 210. Light initially launched intosecondary section 430 may be guided by cladding 420, but this lighttraverses without guiding across primary section 410 and secondarysection 430. When the core size of rotator fiber 220 tapers upwardtoward output fiber 230, a divergence angle of this light decreasesinversely to the core size (as is known for any optical fiber taper),causing more and more of this light to become trapped within primarysection 410 as the divergence angle drops below the NA defined by therefractive index of primary section 410 and a refractive index ofsecondary section 430 interface (i.e., an n₁-n₃ interface). In someimplementations, with a suitable design of rotator fiber 220 and theassociated taper ratio, at least 50% (e.g., 80%) of light launched intosecondary section 430 can be captured by primary section 410 and acquirerotary character.

FIG. 6 is a flow chart of an example process 600 of a spun-fibertechnique for fabricating rotator fiber 220.

As shown in FIG. 6, process 600 may include fabricating a rotator fiber220 preform having a unitary core with a refractive index structure thatangularly varies with respect to a center of the rotator fiber 220preform (block 610). For example, a preform for a fiber cross-sectionalstructure, such as that shown in FIG. 4A, may be fabricated using fourquarter-round pieces of glass of index with refractive index of n₁(e.g., forming primary section 410), at least three plates of glass withrefractive index n₃ (e.g., forming secondary section 430), and a glasstube with refractive index n₂ (e.g., forming cladding 420). Othermethods of fabricating a rotator fiber 220 preform are also possible.

As further shown in FIG. 6, process 600 may include consolidating therotator fiber 220 preform in order to create a consolidated rotatorfiber 220 preform (block 620). In some implementations, the rotatorfiber 220 preform may be consolidated using a heat source (e.g., suchthat the pieces of glass of the rotator fiber 220 preform melttogether). In some implementations, the rotator fiber 220 preform may beconsolidated during the preforming process associated with block 610, orduring a drawing and spinning process associated with block 630described below.

As further shown in FIG. 6, process 600 may include concurrently drawingand spinning the consolidated rotator fiber 220 preform in order tocreate a spun rotator fiber 220 (block 630). In some implementations,the consolidated rotator fiber 220 preform may be secured in a preformspinner on a fiber draw tower, and the consolidated rotator fiber 220preform may be drawn while spinning (e.g., using conventional techniquesassociated with creating a so-called spun fiber) in order to create thespun rotator fiber 220.

In some implementations, a rate of spin relative to a fiber draw speedmay determine a rate of twist in the spun rotator fiber 220. In someimplementations, the rate of spin is selected such that the rate oftwist in the spun rotator fiber 220 is that which is desired for therotary optical beam. Typical rates of twist can be, for example, in arange from approximately 50 rotations per meter to approximately 2000rotations per meter (although slower or faster rates may be used in somecases). In some implementations, the spun rotator fiber 220 is drawndown such that a size of a core (e.g., a diameter of unitary core 405)is approximately equal to, or slightly less than, a size of a core ofoutput fiber 230.

As further shown in FIG. 6, process 600 may include splicing the spunrotator fiber 220 to an end of output fiber 230 (block 640). Forexample, an end of the spun rotator fiber 220 may be fusion-spliced ontoan end of output fiber 230.

As further shown in FIG. 6, process 600 may include tapering the spunrotator fiber 220 in order to create a tapered spun rotator fiber 220,wherein, within the tapered spun rotator fiber 220, the unitary corerotates about an optical axis of the tapered spun rotator fiber 220along a length of the tapered spun rotator fiber 220 (block 650).

In some implementations, a downward taper may be created in the spunrotator fiber 220 using a heat source (e.g., a torch, a fusion splicer,and/or the like) to heat and soften the spun rotator fiber 220 such thatthe size of the core of the spun rotator fiber 220 tapers down to beapproximately equal to or slightly greater than a size of a core ofinput fiber 210. Here, the tapering inherently reduces the rate of twistof the tapered spun rotator fiber 220 (e.g., as illustrated in FIG. 5),such that a rate of twist at an input end of the tapered spun rotatorfiber 220 may be zero or near zero, thus achieving a match to thetypically non-rotary nature of light beam launched by input fiber 210.

As described above, the taper rate may be selected such that transitionsof light propagating through the tapered spun rotator fiber 220 (e.g.,from a first rotational state to a second rotational state) may beadiabatic or near-adiabatic in order to, for example, minimizebrightness loss and/or maximize purity of rotational state(s) generatedin the rotary optical beam by the tapered spun rotator fiber 220. Morespecifically, a rate of increase in size of the core, an increase of therate of twist, and the transfer of light from cladding 420 into unitarycore 405 should be sufficiently gradual to ensure adiabatic transitions.An adiabatic transition can be defined as one in which making thetransition even more gradual does not result in significant performanceimprovement.

In some implementations, after tapering in order to create the taperedspun rotator fiber 220, the tapered spun rotator fiber 220 may bespliced (e.g., fusion-spliced) onto an end of input fiber 210.

As an example, using the previously provided values for a 100 μm corediameter tapered spun rotator fiber 220, a 100 μm core diameter outputfiber 230 could be spliced onto rotator fiber 220, and an input end ofthe tapered spun rotator fiber 220 could be tapered to, for example, a30 μm core diameter in order to match a 30 μm core input fiber 210. Inthis example, a rate of twist can be calculated to be reduced by thistaper to 15 rotations per meter (e.g., (30/100)²×166.7=15 rotations permeter), resulting in a rotational state of approximately 0.18 (l˜0.18)at an input end of rotator fiber 220, which is effectively non-rotaryand is thus well-matched to a non-rotary input optical beam carried byinput fiber 210. In some implementations, the core of input fiber 210may be significantly smaller in size than the core of output fiber 230(e.g., the size of the core of input fiber 210 may be less than or equalto approximately 30% of the size of the core of output fiber 230) sothat rotator fiber 220 will have a zero or near zero rate of twist at aninput end of rotator fiber 220.

Although FIG. 6 shows example blocks of process 600, in someimplementations, process 600 may include additional blocks, fewerblocks, different blocks, or differently arranged blocks than thosedepicted in FIG. 6. Additionally, or alternatively, two or more of theblocks of process 600 may be performed in parallel.

In some implementations, a process of a spun-fiber technique forfabricating rotator fiber 220 may include fabricating a rotator fiberpreform having a unitary core with a refractive index structure thatangularly varies with respect to a center of the rotator fiber preform;consolidating the rotator fiber preform in order to create aconsolidated rotator fiber preform; concurrently drawing and spinningthe consolidated rotator fiber preform in order to create a spun rotatorfiber; and tapering the spun rotator fiber in order to create a taperedspun rotator fiber, wherein, within the tapered spun rotator fiber, theunitary core rotates about an optical axis of the tapered spun rotatorfiber along a length of the tapered spun rotator fiber. In someimplementations, the tapering of the spun rotator fiber creates anadiabatic transition between an input fiber and the output fiber, and anadiabatic transition from a first rotational state to a secondrotational state.

FIG. 7 is a flow chart of an example process 700 of a twisted fibertechnique for fabricating rotator fiber 220.

As shown in FIG. 7, process 700 may include fabricating a rotator fiber220 preform having a unitary core with a refractive index structure thatangularly varies with respect to a center of the rotator fiber 220preform (block 710). For example, the rotator fiber 220 preform may befabricated in a manner similar to that described above in associationwith example process 600.

As further shown in FIG. 7, process 700 may include consolidating therotator fiber 220 preform in order to create a consolidated rotatorfiber 220 preform (block 720). For example, the rotator fiber 220preform may be consolidated in a manner similar to that described abovein association with example process 600.

As further shown in FIG. 7, process 700 may include drawing theconsolidated rotator fiber 220 preform in order to create a drawnrotator fiber 220 (block 730). In some implementations, the consolidatedrotator fiber 220 preform may be drawn using a conventional fiberdrawing process, without spinning. In some implementations, theconsolidated rotator fiber 220 preform may be drawn down such that asize of a core of the drawn rotator fiber 220 (e.g., a size of unitarycore 405) is approximately equal to or slightly less than a size of acore of output fiber 230.

As further shown in FIG. 7, process 700 may include splicing the drawnrotator fiber 220 to an end of output fiber 230 (block 740). Forexample, an end of the drawn rotator fiber 220 may be fusion-splicedonto an end of output fiber 230.

As further shown in FIG. 7, process 700 may include twisting the drawnrotator fiber 220 in order to create a twisted rotator fiber 220,wherein, within the twisted rotator fiber 220, the unitary core rotatesabout an optical axis of the twisted rotator fiber 220 along a length ofthe twisted rotator fiber 220 (block 750).

In some implementations, the drawn rotator fiber 220 may be twistedwhile being heated and/or softened using a heat source (e.g., a torch, afusion splicer, and/or the like) in order to create the twisted rotatorfiber 220 with a variable rate of twist (e.g., a rate of twist thatvaries from zero or near zero at an input end of the twisted rotatorfiber 220 to a desired rate of twist at an output end of the twistedrotator fiber 220). In some implementations, a taper profile can also beimparted to the twisted rotator fiber 220 so that a size of the twistedrotator fiber 220 matches both input fiber 210 and output fiber 230.

In some implementations, after twisting in order to create the twistedspun rotator fiber 220, the twisted rotator fiber 220 may be spliced(e.g., fusion-spliced) onto an end of input fiber 210.

Notably, process 700 may be somewhat more complicated than process 600because of the need to generate a variable twist in the twisted rotatorfiber 220, rather than a constant twist during the draw associated withthe tapered spun rotator fiber 220. However, process 700 may provideadditional degrees of freedom as compared to process 600. For example,process 700 may allow use of an input fiber 210 with a larger core sizethan that of output fiber 230. As another example, process 700 may allowthe rate of twist of the input end of the twisted rotator fiber 220 tobe zero (e.g., rather than near zero) as compared to process 600, wherethe rate of twist at the input end of the tapered spun rotator fiber isdetermined by the taper ratio in the spun-fiber technique. In someimplementations, a hybrid approach is possible, where a spun rotatorfiber 220 is modified by tapering and applying additional variable twistusing a heat source in order to fine-tune (or completely remove) therate of twist at the input end.

Although FIG. 7 shows example blocks of process 700, in someimplementations, process 700 may include additional blocks, fewerblocks, different blocks, or differently arranged blocks than thosedepicted in FIG. 7. Additionally, or alternatively, two or more of theblocks of process 700 may be performed in parallel.

In some implementations, a process of a twisted fiber technique forfabricating rotator fiber 220 may include fabricating a rotator fiberpreform including a unitary core with a refractive index structure thatangularly varies with respect to a center of the rotator fiber preform;consolidating the rotator fiber preform in order to create aconsolidated rotator fiber preform; drawing the consolidated rotatorfiber preform in order to create a drawn rotator fiber; and twisting thedrawn rotator fiber in order to create a twisted rotator fiber, wherein,within the twisted rotator fiber, the unitary core rotates about anoptical axis of the twisted rotator fiber along a length of the twistedrotator fiber. In some implementations, the process may further includesoftening the drawn rotator fiber with a heat source while twisting thedrawn rotator fiber, wherein the drawn rotator fiber is twisted suchthat the twisted rotator fiber has a variable twist rate along thelength of the twisted rotator fiber.

FIGS. 8A-8C are diagrams associated with example simulations usingvarious taper lengths of rotator fiber 220. In the simulationsassociated with FIGS. 8A-8C, input fiber 210 has a 30 μm core and outputfiber 230 has a 100 μm core. Rotator fiber 220 is a spun rotator fibertapering with a parabolic profile from a 30 μm core to a 100 μm core,and has a rate of twist of 166.7 rotations/m at an output end of rotatorfiber 220. Further, input fiber 210 carries six equally populated modes:LP₀₁, LP₀₂, LP₁₁₍₊₎, LP¹¹⁽⁻⁾, LP₂₁₍₊₎, and LP²¹⁽⁻⁾, where (+) and (−)indicate the two possible rotational directions of the correspondingmodes. The LP₁₁ mode and the LP₂₁ mode each have a small amount ofrotation (l=1 and l=2, respectively), but since all six modes areequally populated, the input mode mixture has an average rotation stateof zero. A NA of the cladding of rotator fiber 220 is 0.22 (e.g., so allrelevant modes were strongly guided). A quality of the output opticalbeam is characterized by the number of modes excited.

Rotator fibers 220 of FIGS. 8A, 8B, and 8C are associated with taperlengths of 10 mm, 40 mm, and 80 mm, respectively, in order to assessadiabaticity of these taper lengths. The example simulations showed thatoutput radiation was found to be almost entirely in the form of stronglyrotary modes, LP_(l1), as desired. Results are shown in FIGS. 8A-8C,showing the modal power as a function of rotational number l.

As shown, the rotational states generated by the above described rotatorfiber are centered around l˜18. However, there is some distribution ofstates because more than one input mode was populated. Additionally,based on comparing FIG. 8A to FIGS. 8B and 8C, it can be seen that the10 mm taper has considerably more states excited than the 40 mm taperand the 80 mm taper. This result indicates that the 10 mm taper may betoo short to be adiabatic (i.e., that the 10 mm taper has too abrupt achange in the rotator fiber 220 parameters going from the input end tothe output end), thereby causing additional modes to be excited anddegrading brightness and modal purity.

On the other hand, as indicated by comparing FIGS. 8B and 8C, there isrelatively little change between the 40 mm taper and the 80 mm taper.This indicates that both of these tapers are adiabatic and the resultingmodal distribution is near optimal. Indeed, considering that six inputmodes were populated, in the ideal case six output modes would bepopulated. As can be seen, most of the output population in theadiabatic tapers is indeed captured within about six modes, with someslight spreading into neighboring modes.

Because all of the generated modes are rotary modes, an output spot,associated with an output of rotator fiber 220, may be a clean annuluspattern with a sharp edge, as desired for more effective materialprocessing.

As indicated above, FIGS. 8A-8C are provided merely as examples. Otherexamples are possible and may differ from what is described with regardto FIGS. 8A-8C.

In some implementations, as described above, rotator fiber 220 may notinclude secondary section 430 (i.e., unitary core 405 may include onlyprimary section 410), and a center of unitary core 405 (i.e., a centerof primary section 410) may be offset from the optical axis of rotatorfiber 220 along the length of rotator fiber 220 in association with atleast partially converting an input optical beam to a rotary opticalbeam. FIGS. 9A and 9B are diagrams of an example rotator fiber 220 inwhich a center of unitary core 405 is offset from the optical axis ofrotator fiber 220.

As shown in FIGS. 9A and 9B, in some implementations, unitary core 205may include a single primary section 410, and a center of unitary core405 (i.e., a center of the signal primary section 410) may be offsetfrom the optical axis of rotator fiber 220. Such a unitary core 405 isherein referred to as an offset unitary core 405. In someimplementations, the offset unitary core 405 may have a circular crosssection (as shown in FIGS. 9A and 9B), a rectangular cross section, anelliptical cross section, a ring-shaped cross section, a partial ringshaped cross section, a wedge shaped cross section, or another shape.The example rotator fiber 220 shown in FIGS. 9A and 9B may represent,for example, an 80 μm diameter (highly multimode) unitary core 405(e.g., with 0.22 NA and thousands of modes in a vicinity of a typicaloperating wavelength of approximately 1 μm, 1.5 μm, 1.9 μm, or the like)with a 10 μm axis offset relative to the central axis of cladding 420(e.g., a 400 μm diameter cladding).

As shown in FIGS. 9A and 9B, the offset unitary core 405 may twist alongthe length of rotator fiber 220 (e.g., such that unitary core 405rotates about the optical axis of rotator fiber 220 along the length ofrotator fiber 220). In such a case, the angular variation of therefractive index structure may be defined by the offset of unitary core405 with respect to the optical axis of rotator fiber 220. Here, due tothe offset unitary core 405 twisting about the optical axis, lightpropagating in the offset unitary core 405 is twisted about the opticalaxis of rotator fiber 220 along the length of rotator fiber 220, whichgenerates the rotary optical beam (e.g., including light propagating inat least one rotary guided mode or at least one rotary leaky wave), in amanner similar to that described above. In some implementations, therotary optical beam may have an annular shape at the second end ofrotator fiber 220, as described above.

In some implementations, a rate of twist at which the offset unitarycore 405 twists about the optical axis increases from a first rate oftwist at a first end of rotator fiber 220 to a second rate of twist at asecond end of rotator fiber 220, as described above. In someimplementations, the first rate of twist at the first end of rotatorfiber 220 may be less than or equal to one twist per 50 mm. In someimplementations, as shown in FIG. 9A, unitary core 405 being offset fromthe axis of rotator fiber 220 and being twisted about the axis ofrotator fiber 220 along the length of rotator fiber 220 causes theoffset unitary core 405 to have a helical shape.

In some implementations, rotator fiber 220 including an offset unitarycore 405 (e.g., a single primary section 410 offset from the opticalaxis) may be tapered such that a size of unitary core 405 substantiallymatches a size of a core region of input fiber 210 and/or output fiber230 at respective ends of rotator fiber 220. In some implementations,rotator fiber 220 may be tapered such that a size of rotator fiber 220at the first end of rotator fiber 220 is smaller than a size of rotatorfiber 220 at the second end of rotator fiber 220.

In some implementations, as shown in FIG. 9B, a thickness of cladding420 surrounding the offset unitary core 405 is non-uniform at a givencross-section of rotator fiber 220 (due to the offset of the singleprimary section 410 from the optical axis). Thus, in someimplementations, a cross-section of rotator fiber 220 including offsetunitary core 405 may be asymmetric with respect to the optical axis ofrotator fiber 220.

In some implementations, rotator fiber 220 including an offset unitarycore 405 (e.g., including a single primary section 410) may berelatively simple to fabricate (e.g., as compared to the example rotatorfiber shown in FIG. 4A). For example, a conventional preform with acentered core (e.g., a unitary core 405 comprising a single primarysection 410) can be fabricated, and a portion of cladding 420 can beground off such that unitary core 405 is off-centered. Next, a re-sleeveoperation can be performed (if desired) to add additional claddingmaterial while maintaining the off-axis position of unitary core 405.Alternatively, an undoped rod can be drilled with an off-center hole anda core rod or core/clad rod can be inserted into the off-center hole.This structure can then be consolidated and drawn to fiber. Notably,these processes are provided as examples, and other techniques can beused to fabricate the desired rotator fiber 220 with the offset unitarycore 405.

In some implementations, an output end of the twisted offset unitarycore 405 can be spliced into a suitable conventional, non-helical-coremultimode output fiber 230 in such a way that the rotating character ofthe rotary optical beam is preserved. In some implementations, a radiusof a core of output fiber 230 may match a maximum displacement of theoffset unitary core 405. For example, with an 80 μm diameter, 10μm-offset unitary core 405, a maximum displacement of unitary core 405is 50 μm (e.g., 40 μm radius+10 μm offset=50 μm), and a majority of thelight inside the offset unitary core 405 should be located near that 50μm radius (due to centrifugal force). Therefore, in this example, asuitable matching output fiber 230 would be a 50 μm radius (i.e., 100 μmdiameter) core fiber. Here, the rotating beam should transfer seamlesslyfrom where it is confined at the outskirts of the offset unitary core405 to the 50 μm radius of the core of output fiber 230.

In order to convert a non-rotary input optical beam at least partiallyinto a rotary optical beam with maximum efficiency, the techniquedescribed above can be used. For example, rotator fiber 220 with theoffset unitary core 405 can be tapered down and spliced onto input fiber210. Using the numbers from the above example, if input fiber 210 has a30 μm core, then rotator fiber 220 with the 80 μm offset unitary core405 and 400 μm cladding 420 can be tapered down to a 30 μm core, 150 μmcladding. Input fiber 210 can be spliced onto rotator fiber 220 suchthat the core of input fiber 210 and the offset unitary core 405 arealigned. Here, since input fiber 210 may have a centered core, outeredges of the cladding of input fiber 210 and cladding 420 of rotatorfiber 220 may not be aligned when the cores are aligned. As in the aboveexample, output fiber 230 could have a 100 μm core. For ease ofsplicing, cladding of output fiber 230 could also be 400 μm, and thecladding of output fiber 230 and cladding 420 of rotator fiber 220 maybe aligned.

Further, as described above, a pitch of the fiber rotation can be chosento provide a desired beam rotation rate (which corresponds to aparticular output NA), and a length of the taper can be chosen optimizeoutput quality. For the above-described example, using a 6 mm rotationpitch, an 80 mm linear taper length is predicted to give a high-qualityrotary beam for an input mode mix of 50% LP₀₁, 50% LP₀₂. Here, in boththe near and far fields, a clear ring structure may be formed, meaningthat a high proportion of rotary modes are excited. Modeling shows thatrotational numbers of the excited modes range from approximately 8 toapproximately 20, in agreement with modeled results described above.

Furthermore, as described above, the use of a tapered structuresimultaneously provides a gradual transition from a non-rotary to arotary state and a gradual transition from a smaller (e.g., 30 μm) inputoptical beam to a larger (e.g., 100 μm annular) output beam. In someimplementations, a square-root taper pattern may be utilized.

In some implementations, rotator fiber 220 may be fabricated using aspun fiber technique (e.g., spinning a preform during a fiber draw) andthen applying a non-spun taper, or can be fabricated by using a non-spunfiber and implementing a rotation during a taper process, as describedabove.

As with any rotary beam, care needs to be taken to avoid significantbend loss. Thus, the NA of cladding 420 of rotator fiber 220 includingthe offset unitary core 405 may be higher than the divergence of therotary light generated. In the above example with a 6 mm pitch and a 100μm output fiber 230, an output divergence (far-field radius) extends toapproximately 0.10 radians. Here, using fibers with NA of 0.12 orgreater (e.g., 0.15 or greater) should provide adequate margin toprevent bend loss.

Notably, the effect provided by offset unitary core 405 of rotator fiber220 need not utilize a circular core. For example, one otherimplementation includes, starting with the “four-bladed” embodimentshown in FIG. 4A, and injecting light into only one of the primarysections 410. In this case there is effectively only a single primarysection 410 carrying light, and this single primary section 410 isoffset from the center of rotator fiber 220 (e.g., with a wedge apexbeing near the center and all the light-carrying area being away fromthe center).

As indicated above, FIGS. 9A and 9B are provided merely as examples.Other examples may differ from what is described with regard to FIGS. 9Aand 9B.

FIG. 10 is a flow chart of an example process 1000 of a first techniquefor fabricating rotator fiber 220 including an offset unitary core 405.

As shown in FIG. 10, process 1000 may include obtaining a fiber preformincluding a core and a cladding surrounding the core, the core beingapproximately centered on a central axis of the fiber preform (block1010). For example, a fiber preform including a circular core and acladding surrounding the core may be obtained. Here, the core of thefiber preform may be approximately centered on a central axis of thefiber preform.

As further shown in FIG. 10, process 1000 may include removing a portionof the cladding surrounding the core along a length of the fiber preform(block 1020). For example, a portion of the cladding of the fiberpreform may be ground off along the length of the fiber preform.

As further shown in FIG. 10, process 1000 may include re-sleeving thefiber preform in order to create a rotator fiber preform, wherein,within the rotator fiber preform, the center of the core is offset fromthe central axis of the rotator fiber preform (block 1030). For example,a re-sleeve operation can be performed to add cladding material to thefiber preform, while maintaining an off-axis position of the corerelative to a (shifted) central axis of the fiber preform.

As further shown in FIG. 10, process 1000 may include creating rotatorfiber 220 using the rotator fiber preform, wherein, within rotator fiber220, a center of the core (e.g., unitary core 405) core is offset fromthe axis of rotator fiber 220 along a length of rotator fiber 220, andwherein, within rotator fiber 220, the offset unitary core 405 twistsabout the axis of rotator fiber 220 along the length of rotator fiber220 (block 1040). In some implementations, rotator fiber 220 may becreated using a spun fiber technique (e.g., including a concurrent drawand spin), as described above in association with FIG. 6. In someimplementations, rotator fiber 220 may be created using a twisted fibertechnique (e.g., including a draw and then a twist), as described abovein association with FIG. 7.

Although FIG. 10 shows example blocks of process 1000, in someimplementations, process 1000 may include additional blocks, fewerblocks, different blocks, or differently arranged blocks than thosedepicted in FIG. 10. Additionally, or alternatively, two or more of theblocks of process 1000 may be performed in parallel.

FIG. 11 is a flow chart of an example process 1100 of a second techniquefor fabricating rotator fiber 220 including an offset unitary core 405.

As shown in FIG. 11, process 1100 may include creating an opening alonga length of a cladding rod, the opening being offset from a central axisof the cladding rod (block 1110). For example, an off-center hold can bedrilled into an (undoped) cladding rod.

As further shown in FIG. 11, process 1100 may include inserting a corerod in the opening along the length of the cladding rod (block 1120).For example, a core rod can be inserted in the hole in the cladding rodalong the length of the cladding rod.

As further shown in FIG. 11, process 1100 may include consolidating thecore rod and the cladding rod in order to create a consolidated rotatorfiber preform (block 1130). For example, the core rod and the claddingrod may be consolidated to form a preform of rotator fiber 220. In someimplementations, the rotator fiber preform may be consolidated using aheat source (e.g., such that the pieces of glass of the rotator fiber220 preform melt together). In some implementations, the rotator fiberpreform may be consolidated during creating of rotator fiber 220associated with block 1140 described below.

As further shown in FIG. 11, process 1100 may include creating rotatorfiber 220 using the rotator fiber preform, wherein, within rotator fiber220, a center of the core (e.g., unitary core 405) core is offset fromthe axis of rotator fiber 220 along a length of rotator fiber 220, andwherein, within rotator fiber 220, the offset unitary core 405 twistsabout the axis of rotator fiber 220 along the length of rotator fiber220 (block 1140). In some implementations, rotator fiber 220 may becreated using a spun fiber technique (e.g., including a concurrent drawand spin), as described above in association with FIG. 6. In someimplementations, rotator fiber 220 may be created using a twisted fibertechnique (e.g., including a draw and then a twist), as described abovein association with FIG. 7.

Although FIG. 11 shows example blocks of process 1100, in someimplementations, process 1100 may include additional blocks, fewerblocks, different blocks, or differently arranged blocks than thosedepicted in FIG. 11. Additionally, or alternatively, two or more of theblocks of process 1100 may be performed in parallel.

Some implementations described herein provide an optical fiber devicefor generating an optical beam with an annular beam shape directly in anoptical fiber (i.e., without any free-space optics). More specifically,the generated optical beam is a rotary optical beam (i.e., an opticalbeam that propagates in the optical fiber in a helical direction),thereby generating an optical beam with an annular beam shape. In someimplementations, the rotary character of the optical beam can bepreserved (e.g., when the optical beam exits the optical fiber) suchthat a laser spot projected from the optical fiber onto a workpiece, forexample, shows an annular beam profile with sharp edges and high beamquality.

The foregoing disclosure provides illustration and description, but isnot intended to be exhaustive or to limit the implementations to theprecise form disclosed. Modifications and variations are possible inlight of the above disclosure or may be acquired from practice of theimplementations.

For example, rotator fiber 220 has been described as being used for thepurpose of converting a non-rotary optical beam into a rotary opticalbeam. However, in some applications, rotator fiber 220 may be used in areverse direction in order to convert an input rotary optical beam intoan output non-rotary optical beam. This may be achieved by reversing thedesign of rotator fiber 220, including the taper and the variation oftwist, so that the rate of twist at an input end of rotator fiber 220matches the rotation of the input optical beam, and so that the rate oftwist at an output end of rotator fiber 220 is zero or near zero. Eitherof the fabrication techniques described above can be adapted to thisexample.

As another example, rotator fiber 220 may be designed in order toconvert an input optical beam with any rotational state into an outputoptical beam with another (i.e., different) rotational state. Thecriterion for achieving this is that the rate of twist at the input endof rotator fiber 220 should match the rotational state of the inputoptical beam, and the rate of twist at an output end of rotator fiber220 should match the desired rotational state of the output opticalbeam. Either of the fabrication techniques described above can beadapted to this example.

Even though particular combinations of features are recited in theclaims and/or disclosed in the specification, these combinations are notintended to limit the disclosure of possible implementations. In fact,many of these features may be combined in ways not specifically recitedin the claims and/or disclosed in the specification. Although eachdependent claim listed below may directly depend on only one claim, thedisclosure of possible implementations includes each dependent claim incombination with every other claim in the claim set.

No element, act, or instruction used herein should be construed ascritical or essential unless explicitly described as such. Also, as usedherein, the articles “a” and “an” are intended to include one or moreitems, and may be used interchangeably with “one or more.” Furthermore,as used herein, the term “set” is intended to include one or more items(e.g., related items, unrelated items, a combination of related items,and unrelated items, etc.), and may be used interchangeably with “one ormore.” Where only one item is intended, the term “one” or similarlanguage is used. Also, as used herein, the terms “has,” “have,”“having,” or the like are intended to be open-ended terms. Further, thephrase “based on” is intended to mean “based, at least in part, on”unless explicitly stated otherwise.

What is claimed is:
 1. An optical fiber device, comprising: a core section that twists about an axis of the optical fiber device along a length of the optical fiber device, wherein a center of the core section is offset from the axis of the optical fiber device along the length of the optical fiber device, wherein a rate of twist at which the core section twists about the axis increases from a first rate of twist at a first end of the optical fiber device to a second rate of twist at a second end of the optical fiber device, and wherein the core section being twisted about the axis is to cause an optical beam, launched at the first end of the optical fiber device, to be at least partially converted to a rotary optical beam at a second end of the optical fiber device; and a cladding surrounding the core section.
 2. The optical fiber device of claim 1, wherein a thickness of the cladding surrounding the core section is non-uniform at a cross-section of the optical fiber device.
 3. The optical fiber device of claim 1, wherein a cross-section of the optical fiber device is asymmetric with respect to the axis of the optical fiber device.
 4. The optical fiber device of claim 1, wherein the core section is a circular core section.
 5. The optical fiber device of claim 1, wherein the core section being offset from the axis of the optical fiber device and being twisted about the axis of the optical fiber device along the length of the optical fiber device causes the core section to have a helical shape.
 6. The optical fiber device of claim 1, wherein a core of the optical fiber device includes only the core section.
 7. The optical fiber device of claim 1, wherein the optical fiber device includes a unitary core comprising a plurality of core sections including the core section.
 8. The optical fiber device of claim 1, wherein the first rate of twist at the first end of the optical fiber device is less than or equal to one twist per 50 millimeters.
 9. The optical fiber device of claim 1, wherein the optical fiber device is tapered such that a size of the optical fiber device at the first end of the optical fiber device is smaller than a size of the optical fiber device at the second end of the optical fiber device.
 10. The optical fiber device of claim 1, wherein the rotary optical beam has an annular shape at the second end of the optical fiber device.
 11. The optical fiber device of claim 1, wherein the rotary optical beam includes light propagating in at least one rotary guided mode or at least one rotary leaky wave.
 12. A method, comprising: receiving, by a rotator fiber, an optical beam at a first end of the rotator fiber, wherein the rotator fiber includes a core section that twists about an axis of the rotator fiber along a length of the rotator fiber such that a center of the core section is offset from the axis of the rotator fiber along the length of the rotator fiber, wherein a rate of twist at which the core section twists about the axis increases from a first rate of twist at a first end of the rotator fiber to a second rate of twist at a second end of the rotator fiber; at least partially converting, by the rotator fiber, the optical beam to a rotary optical beam, wherein the optical beam is at least partially converted to the rotary optical beam as a result of the core section being twisted about the axis; and outputting, by the rotator fiber, the rotary optical beam.
 13. The method of claim 12, wherein a thickness of a cladding surrounding the core section is non-uniform at a cross-section of the rotator fiber.
 14. The method of claim 12, wherein a cross-section of the rotator fiber is asymmetric with respect to the axis of the rotator fiber.
 15. The method of claim 12, wherein the core section is a circular core section.
 16. The method of claim 12, wherein the core section being offset from the axis of the rotator fiber and being twisted about the axis of the rotator fiber along the length of the rotator fiber causes the core section to have a helical shape.
 17. The method of claim 12, wherein a core of the rotator fiber includes only the core section.
 18. The method of claim 12, wherein the first rate of twist at the first end of the rotator fiber is less than or equal to one twist per 50 millimeters.
 19. An annular beam generator, comprising: an optical fiber device including: a core section that twists about an axis of the optical fiber device along a length of the optical fiber device, the core section being offset from the axis of the optical fiber device along the length of the optical fiber device, wherein a rate of twist at which the core section twists about the axis increases along the length of the optical fiber device from a first end of the optical fiber device to a second end of the optical fiber device; and a cladding surrounding the core section.
 20. The annular beam generator of claim 19, wherein the core section being twisted about the axis is to cause an optical beam, launched at the first end of the optical fiber device, to be at least partially converted to a rotary optical beam at a second end of the optical fiber device. 